The present invention relates to a light-beam scanning apparatus and an image forming apparatus, both for use in digital copy machines, laser printers or the like in which the photosensitive drum is scanned with a laser beam to form a latent image on the photosensitive drum.
To increase the image-forming speed, digital copy machines and the like have been developed, which comprise a light beam scan apparatus of multi-beam type. The light beam scan apparatus generates a plurality of light beams and scans a plurality of lines with these light beams at the same time.
Digital copy machines comprising an optical section such as a light beam scan apparatus are known. In each of the copy machines, the positions of the light beams are controlled in two modes to form high-quality images. First, the positions where the light beams are applied are controlled in the scanning direction (i.e., control of beam positions in the main scanning direction). Second, the positions where the light beams pass are controlled (i.e., control of beam positions in the sub-scanning direction).
In the conventional beam-position sensor, however, two timing sensors that detect the start and end, respectively, are identical in the sub-scanning direction. Therefore, they may malfunction at the boundary between their effective detection region and non-effective detection region. Consequently, the light beams may not be controlled to pass at desired positions. Further, the beam-position sensor cannot have the desired sensitivity, because the components of its beam-detecting section have characteristics different from the design values. (The photodiode has sensitivity different from the designed value, the operational amplifier has an offset-voltage and -resistance different from the design values, and the capacitor has a capacitance difference form the designed value.)
A light beam scanning apparatus according to the invention is designed to scan a surface with a light beam. The apparatus comprises: a light-emitting unit for emitting a light beam; a scanning unit for scanning the surface with the light beam emitted from the light-emitting unit, in a main scanning direction; first beam-detecting unit which is provided on the surface or at a position equivalent to the surface and which converts the light beam to an electric signal at a first position in the main scanning direction; a second beam-detecting unit which is provided on the surface or at a position equivalent to the surface and which converts the light beam to an electric signal at a second position in the main scanning direction, the second position being downstream the first beam-detecting unit; a third beam-detecting unit which is provided on the surface or at a position equivalent to the surface, which has a length different from the first beam-detecting unit in a sub-scanning direction and which converts the light beam to an electric signal at a third position in the main scanning direction, the third position being downstream the second beam-detecting unit; and control unit which reads beam information from the second beam-detecting unit at the time the first and third beam-detecting units detect the light beam and controls a position where the light beam passes or intensity of the light beam.
This invention provides an image forming apparatus that forms an image on an image-forming medium. The apparatus comprises: a light-emitting unit which emits a light beam; an image-carrying body on which a latent image is formed by using the light beam emitted from the light-emitting unit; a scanning unit which scans the surface with the light beam emitted from the light-emitting unit, in a main scanning direction; first beam-detecting unit which is provided on the surface or at a position equivalent to the surface and which converts the light beam to an electric signal at a first position in the main scanning direction; a second beam-detecting unit which is provided on the surface or at a position equivalent to the surface and which converts the light beam to an electric signal at a second position in the main scanning direction, the second position being downstream the first beam-detecting unit; a third beam-detecting unit which is provided on the surface or at a position equivalent to the surface, which has a length different from that of the first beam-scanning unit in a sub-scanning direction and which converts the light beam to an electric signal at a third position in the main scanning direction, the third position being downstream the second beam-detecting unit; control unit which reads beam information from the second beam-detecting unit at the time the first and third beam-detecting units detect the light beam and controls a position where the light beam passes and intensity of the light beam; and an image-forming unit which causes the scanning unit to scan the surface of the image-carrying body with the light beam from the light-emitting unit when the control unit finishes controlling the position and intensity of the light beam, thereby forming an latent image, and which converts the latent image to a visible image and transfers the visible image onto the image-forming medium.